(A) Field of the Invention
This invention relates to the installation of a heat shrinkable polymer roll cover over the lateral area of a process roll. The invention further relates to the heat shrinkable sleeve and the process roller covered with the sleeve.
(B) History of the Prior Art
Process rollers or rolls are used in industry to press, roll or convey sheet or laminar materials. Examples of such rolls are nip rolls for the production of plastic sheets and films, paper machine dryer drums used in the production of paper and textile dyeing and drying drums used in the production of textiles.
Originally, such process rolls had a metallic surface. These surfaces were sometimes covered with a polymer film to obtain better corrosion resistance or better abrasion resistance. In addition, such metallic surfaces are now known to have high surface energy characteristics which frequently causes the material being processed to stick to the roll. In most processes, such sticking or adhesion cannot be tolerated.
In order to reduce the surface energy of such process rolls, the rolls were frequently covered with a fluorinated polymer as soon as such polymers became available for such applications.
One of the better methods for covering such rolls in the prior art consisted of placing the roll within a radially elongated fluorinated polymer sleeve which was then heat shrunk to provide a tight fitting roll cover.
Such covers, however, continued to have serious problems since, especially in large rollers, the cover expanded in the axial direction when the roll was heated thus causing uneven roller surface characteristics. Furthermore, especially in larger rollers, the sleeve was able to move on the roller, particularly at high temperatures, which also caused uneven roller surface characteristics.
In order to avoid axial elongation when it was required to heat a roller, the sleeves were axially stretched at the time of application of the sleeve to the roller. Such axial stretching at the time of applying the sleeve to the roller made the application of the sleeve exceedingly difficult and required complicated processes and apparatus. Examples of such processes and apparatus to axially stretch a sleeve during application to a roller are disclosed in U.S. Pat. Nos. 3,426,119 and 3,749,621. Furthermore, such sleeves continued to be free to move on the roller thus creating uneven surface characteristics.
In order to prevent such movement, various adhesives were tried which, while being somewhat successful at low temperatures, were not completely successful at high processing temperatures such as those encountered in paper drying. Certain adhesives were, for example, tried and were found to become brittle at high temperatures, i.e., above about 150.degree. C., and were found to have insufficient adhesion to the fluorinated polymer sleeve.
U.S. Pat. No. 3,481,805 discloses that a silicone resin adhesive could be used when the inside surface of a sleeve was preliminarily etched to aid the adhesive. The patent does not, however, disclose the axial stretching of the sleeve during its application to a roller. Furthermore, the silicone resin adhesive was still insufficiently adherent to a fluorinated polymer sleeve to prevent the sleeve from moving under high temperature conditions.
As a result of the foregoing disadvantages, no commercially available prior art fluoropolymer covered roller was able to operate for an extended period at temperatures in excess of 120.degree. C.